The present invention relates to a sequential control device with rotating drum for engaging gears in a mechanical gearbox of a motor vehicle, particularly of a motor car.
Mechanical gearboxes used in motor vehicles are generally of the type with permanently engaged gears, i.e. of the type in which driving gear wheels carried by one or more primary shafts, or input shafts, permanently mesh with corresponding driven gear wheels carried by one or more secondary shafts, or output shafts, and in which the driving gear wheels are rigidly connected for rotation with the primary shaft (or primary shafts) while the driven gear wheels are mounted idly on the secondary shaft (or secondary shafts). A gear is engaged by coupling the idle gear wheel of the gear train corresponding to the gear to be engaged for rotation with the corresponding shaft. For this purpose, each idle gear wheel, or each pair of adjacent idle gear wheels, is associated with a synchronizing device serving to synchronize the angular velocities of the idle gear wheel to be engaged and of the associated shaft, before the engagement operation is completed to transmit torque through the idle gear wheel in question. A synchronizing device typically comprises a driving part, rigidly connected for rotation by a splined coupling with the corresponding shaft of the gearbox; at least one synchronizing ring forming a conical surface which engages with a corresponding conical surface of the associated idle gear wheel; and a sleeve which is rigidly coupled for rotation with the driving part, and therefore to the corresponding shaft of the gearbox, and which can be moved axially in one or other direction by means of a corresponding engagement fork.
To enable the driver to control an ordinary mechanical gearbox sequentially, control devices for engaging the gears are known, which comprise a rotating drum placed parallel to the shafts of the gearbox and having on its outer cylindrical surface one or more guide grooves in which a plurality of pins engage slidably, each of these pins being rigidly connected for translation with a corresponding engagement fork of the gearbox. The rotation of the drum about its own axis is driven by a drive unit according to commands given by the driver through intermediate control devices. The drive group typically includes an electric motor or a servo-assisted hydraulic device, coupled to a reduction gear unit if necessary. The guide grooves of the drum are shaped so as to produce, as a result of the rotation of the drum, a selective movement of the pins and consequently of the engagement forks, according to predetermined operating modes, thus causing each time the engagement or disengagement of one or more gears.
A sequential control device with rotating drum for engaging the gears in a gearbox of a motor vehicle is known from DE 102 59 485 A1. According to a first embodiment, the control device comprises two synchronizing sleeves associated with a double engagement fork which is guided and supported directly on the drum. The engagement fork is provided with two pins, which engage slidably in a guide groove provided on the outer cylindrical surface of the drum, and which are located on opposite sides of the synchronizing sleeves with respect to the axis of the drum. Since the engagement fork is mounted directly on the drum, instead of on a suitable rod, the number of components of the control device can be reduced. On the other hand, the opposing arrangement of the control pins with respect to the synchronizing sleeves causes the engagement fork to be subject to high overturning torques during operation, and to tend therefore to become inclined with respect to a plane perpendicular to the axes of the drum and of the synchronizing sleeves. According to a second embodiment, the engagement fork is guided and supported on a guide rod, and the two pins are interposed between the axis of the drum and the axes of the synchronizing sleeves. In this case, the overturning torques acting on the engagement fork during operation are smaller, since the drive forces exchanged between the pins and the guide groove act with a lower lever arm. On the other hand, this second embodiment requires the mounting of the engagement fork on a special rod, and therefore requires an additional component as compared with the first embodiment.
GB 208 370 discloses a sequential control device for engaging gears in a mechanical gearbox of a motor vehicle, the device comprising: a drum mounted rotatably about an axis of rotation and having a plurality of guide grooves on its outer surface; and a corresponding plurality of engagement forks, each including a supporting portion supported slidably on the outer surface of the drum, a drive portion designed to shift a corresponding gear wheel of the gearbox, and a pin engaging in a corresponding guide groove of the drum. The guide grooves are shaped so as to produce, as a result of the rotation of the drum, a selective movement of the pins, together with the corresponding engagement forks, parallel to the axis of rotation of the drum to cause each time the engagement of a given gear. The pins are positioned, with respect to the axis of rotation of the drum, on the same side of the drive portion of the engagement forks.